Cardiac pacemakers are usually identified by three letters, of which the first letter identifies the stimulated chamber, the second letter identifies the perceiving chamber (sensing) and the third letter identifies a pacemaker operating mode. The present invention primarily concerns VDD or DDD pacemakers. VDD pacemakers are suitable for stimulating the ventricle of a heart, recording electrical signals both in the ventricle and also in the atrium, and operating both in an inhibiting and also in a triggered mode. DDD pacemakers are additionally capable of also stimulating the atrium of a heart.
Such pacemakers usually involve an atrial refractory time in which electrical signals in the atrium of a heart are either not detected at all or detected signals are not subjected to further processing. Typical signals to be detected, both in the atrium and also in the ventricle of a heart, are atrial or ventricular contractions respectively of the heart, which involve depolarization of the cardiac tissue, which is electrically detectable.
In response to a detected and processed electrical signal in the atrium of a heart, an AV time is usually triggered, at the end of which an electrical stimulation pulse is delivered to the ventricle of the heart if natural contraction of the ventricle is not detected during the AV time.
The time co-ordination of an atrial contraction which is followed by a ventricular contraction is of great significance in regard to hemodynamic efficiency, that is to say the pump efficiency of a heart. Firstly there is a contraction of the atrium which is followed at a given distance by a contraction of a ventricle. After a further time, the VA time, a contraction of the atrium is again due, which is followed after a further AV time again by a contraction of the ventricle, and so forth.
In regard to stimulation in particular of the ventricle, it is necessary to avoid that it does not excessively quickly follow a preceding natural or stimulated contraction of the ventricle as otherwise by virtue of electrical stimulus conduction in the cardiac tissue (myocardium), fibrillation of the ventricle can occur, which results in failure of the pump activity of the heart and can end in death if it is not terminated in good time.
It is further possible for ventricular stimulation events or also natural contractions of the ventricle to be perceived by way of an electrical remote field action in the atrium as electrical signals. Such remote field perception can also result in pacemaker-induced tachycardia.
Finally there can also be retrograde (rearward directed) stimulus conduction from the ventricle to the atrium, which results in premature contraction of the atrium (PAC: premature atrial contraction). If a cardiac pacemaker starts the AV interval by virtue of such a premature atrial contraction and at the end thereof triggers ventricular stimulation, that in turn can result in a further PAC so that the situation then involves pacemaker-induced tachycardia, an excessively increased heart rate.
In order to prevent this, pacemakers usually have a postventricular atrial refractory period (PVARP=Postventricular Atrial Refractory Period). That interval is triggered after a ventricular stimulation event and has the effect that atrial sense events perceived during the PVARP interval are not subjected to further processing for triggering an AV interval. It is known for the PVARP interval to be prolonged after premature ventricular contractions (PVC) or ventricular extrasystoles (VES) in order to avoid pacemaker-induced tachycardia (PMT).
It is possible that natural atrial events are not perceived in particular during a prolonged PVARP. The consequence is that a natural ventricular contraction which follows a non-perceived atrial contraction is perceived not as a natural ventricular contraction but as a ventricular extrasystole and so forth. That phenomenon is known as VES-lockin. VES-lockin can occur both in the VDD mode of operation and also in the DDD mode of operation in the case of pacemakers with a conventional atrial refractory time concept (relatively long PVARP), predominantly at relatively high stimulation rates, if a patient at least temporarily has a relatively slow overconduction of atrial stimuli to the ventricle (AV-overconduction).
A VES-lockin occurs if a premature atrial sense event occurs in an atrial refractory time after a ventricular event, for example in the case of:                premature atrial contractions with sinus node reset during PVARP after a ventricular stimulation event        premature atrial contraction with sinus node reset during PVARP after a ventricular sense event        premature or regular atrial contraction during a prolonged PVARP after a premature ventricular contraction (the above-depicted case)        if a pacemaker is in the Wenckebach condition in which ventricular stimulation does not occur at the end of the AV interval if at that moment the “upper tracking interval” started with all ventricular sense and stimulation events is not yet concluded but ventricular stimulation is displaced to the end of the upper tracking interval. Here, by virtue of the frequent asynchronicity of atrial and ventricular events, it is possible that a regular atrial sense event occurs in the PVARP (postventricular atrial refractory time) after ventricular stimulation and is (slowly) naturally overconducted. Such ventricular contraction which is based on natural overduction is then classified as ventricular extrasystoles VES.        
Besides the specified sources of VES-lockin there are others.
In connection with the prevention of pacemaker-induced tachycardias (PMT) and the prolongation, which serves for that purpose, of the PVARP after a ventricular extrasystole, it happens that a premature atrial contraction or an atrial sense event in the heart is overconducted slowly from the atrium to the ventricle and, in the ventricle, after expiry of the overconduction time provided in the pacemaker, causes ventricular depolarization and contraction of the ventricle, which is then classified by the pacemaker as a ventricular extrasystole. That in turn results in a prolongation of the PVARP.
The next antegrade excitation of the atrium occurs in the prolonged PVARP and, due to natural stimulus conduction from the atrium to the ventricle, triggers the next contraction of the ventricle which is again classified as a ventricular extrasystole. That behavior on the part of the cardiac pacemaker is represented outwardly as a temporary loss of detection of atrial events, atrial sensing.
From the point of view of the patient, longer AV times signify a worsening in ventricle filling as the valves between the atrium and the ventricle close prematurely. That is problematical in the case of some patients. In addition depolarization of the atrium (P-wave) at higher frequencies can occur in a ventricular systole, with the consequence that patients suffer from atrium blockages involve a potential fibrillation risk, as in the case of a pacemaker syndrome. Such a VES-lockin is only terminated when the spontaneous (natural) atrial interval is longer than the PQ interval which extends from an atrial depolarization (P-wave) to a ventricular depolarization (Q-peak), plus the PVARP, or if a main timer of a cardiac pacemaker, which is started with a ventricular extrasystole, expires prior to the next ventricular sense event.
The object of the present invention is to provide as extensive a remedy as possible in respect of the above-depicted problems.